This invention relates to a protective relay device of an electric power system.
Various static protective relay devices utilizing transistors or the like have been used to protect against such faults as short circuits and groundings occurring in an electric power system. As is well known to those skilled in the art, the static relay device misoperates when the input electric quantity contains a DC component or a high frequency component. To prevent such misoperation it has been necessary to insert a bandpass filter which passes only the fundamental frequency of the electric power system in the input circuit of the static relay device. However, when the input electric quantity decreases rapidly, the output from the bandpass filter attenuates over a long time at a frequency near the fundamental frequency of the electric power system. In order to prevent such misoperation caused by the transient phenomenon it is necessary to extend the operation or response time of the relay device. Especially, grounding relays responsive to zero phase currents or inverse phase currents, and differential relays utilized to protect generators or transformers have high sensitivities so that these relays do not operate until the filter output has attenuated sufficiently, thus greatly increasing the operating time of the relays.
Difficulties caused by the use of bandpass filters also occur in overcurrent relays, distance relays and phase comparison relays.
More particularly, when a section of a transmission line is protected by phase comparison relays, the output electric quantities of the bandpass filters located at the opposite ends of the section are applied to first and second level detectors respectively. When the output electric quantity at one end is larger than the detection level L.sub.1 of the first detector at that end, the output electric quantity is sent to the other end through a transmitter. When the output electric quantity at the other end is larger than the output level L.sub.2 of the second level detector at the other end, this electric quantity is applied to the phase comparison circuit at the other end which compares this electric quantity with the signal sent from the one end (a signal having a level larger than detection level L.sub.1) to produce an output signal proportional to the overlapping angle (overlapping time) of the two signals. When a fault occurs in the protected section, the phases of both signals are substantially equal with the result that the overlapping angle becomes larger than a prescribed value necessary to trip circuit breakers thus producing a signal for tripping the same. When there is not fault on the transmission line, or in the case of an external fault, the phase difference between the two inputs to the phase comparison relay is nearly 180.degree. so that the phase comparing circuit does not produce any tripping signal for the circuit breakers. However, when the external fault is cleared and as the electric quantities which have passed through the bandpass filters decrease rapidly the bandpass filters produce outputs which attenuate in an oscillatory manner as has been pointed out hereinabove. If the characteristics (the time constants at the time of discharge) of the bandpass filters at both ends do not coincide with each other the phase difference between the two outputs would become larger. Consequently, the phase difference between two signals applied to the phase comparing circuit gradually decreases. Finally, the two signals will come to overlap for more than a prescribed interval thereby producing a tripping signal for the circuit breakers.
This phenomenon becomes more remarkable as the current of the external fault increases.